When I built my new home a couple years ago, I found that all of the electrical outlets had built-in safety covers (required by local building code). When you look at the outlet, the slots for the plugs are covered and when you plug into the outlet, the covers move out of the way. Have you tried to insert a plug into the outlet?

I think the intent was to use the singles on the inverter circuit but they end up on shore power circuits sometimes. That's what happened to mine - I had them changed at the factory when I was there for a couple of other minor items.

Note that when looking inside the receptacle, you will see that by knocking our the thin pieces of plastic filling the plug blade spaces, it turns into a typical duplex outlet with two plug in spaces. Airstream is trying to limit what can be plugged into their factory installed 1,000 watt inverter which can NOT even power any of the microwave units they install.

Our 2013 25FB International Serenity had the 600 watt inverter of the time and it was truly useless for anything other than the television and DVD player.

We omitted the factory inverter in our Classic as we installed a 2,800 watt Magnum MS-2812 inverter along with a 600 amp-hour lithium battery. It can power the air conditioner.

We removed the factory 1,000 watt inverter in the 23D and replaced it with a 3,000 watt hybrid Magnum MSH-3012 along with a 300 amp-hour lithium battery. The Magnum will pull power from the battery and augment a Honda 2,000 watt generator input to start the air conditioner. The 2,000 watt Honda can keep the air conditioner running once it is started.

The only reason I can think of is the outlet is connected to a single circuit 20a breaker.
This might be the case if the circuit is for a water heater or space heater or other appliance that draws more than 1600w.
NEC 551.43

Outlets are not designed to limit current on a given circuit. If only a single plugin was required they would have installed a none duplex receptacle.

How about a picture.

Obviously outlets are not designed to limit current on a given circuit.

However, when you design a circuit, it is required that you limit the current in each branch circuit by the using appropriate outlet devices.

In the case of a high load appliance a single outlet branch circuit is required, and the required receptacle is a single outlet. Using a duplex receptacle with two outlets changes the code definition of the branch circuit.

Either a duplex outlet with one end blank, or a single outlet in the yoke is acceptable.